This invention is silicone rubber compositions for application as electrical insulation (referred to below as EI silicone rubber compositions). More specifically, this invention relates to an EI silicone rubber composition that is particularly well adapted for application as electrical insulation because after its cure it exhibits an excellent water resistance and also excellent electrical properties, for example, volume resistivity, dielectric constant, dissipation factor, tracking resistance, arc resistance, and erosion resistance.
Compositions made by blending aluminum hydroxide powder into silicone rubber compositions are already known. It is also known that silicone rubbers with excellent electrical properties can be obtained by the cure of silicone rubber compositions of this type (Japanese Patent Publication Kokoku Number Hei 5-12805 (12,805/1993) and Japanese Patent Application Kokai Number Hei 7-57574 (57,574/1995)). However, these silicone rubber compositions contain large amounts of the aluminum hydroxide powder, which is a strongly water absorptive hydrate, and as a result have poor water resistance. These compositions absorb moisture with the passage of time, leading to a decline in their electrical insulating properties, and hence are not fully acceptable as an EI silicone rubber composition for high-voltage use.
The present invention was achieved as a result of extensive research by the present inventors directed to solving the problems described above. In specific terms, the object of the present invention is to provide a silicone rubber composition that cures into a highly water-resistant silicone rubber that has excellent electrical properties.